Receptors for the Fc portions of immunoglobulins are important in triggering many of the protective functions of monocytes, macrophages and polymorphonuclear cells. Receptors for IgG (Fc.gamma. receptors or Fc.gamma.R) on these cells have been extensively investigated and bispecific molecules targeting these receptors have been constructed. (See e.g. European Patent No. 0 255 249 entitled "Monoclonal Antibodies to Fc Receptor for Immunoglobulin G on Human Mononuclear Phagocytes", which is co-owned by Applicants.) In addition, clinical trials of bispecific molecules which have specificity for the Fc.gamma.R and the HER-2/neu antigen, which is found on breast or ovarian cancers, indicate that these molecules are both safe and efficacious (Valone, Frank H. et al. 1995, J. of Clin. Oncol. 13(9): 2281-2292).
IgA receptors Fc.alpha. receptors (Fc.alpha.R or CD89) are also capable of promoting effector cell function. Binding of ligand to Fc.alpha.R triggers phagocytosis and and antibody mediated cell cytotoxicity in leukocytes and Fc.alpha.R-bearing cell lines. Fc.alpha.receptors can also cooperate with receptors for IgG on effector cells in enhancing the phagocytosis of target cells. Monoclonal antibodies of the IgM (Shen, L. et al., 1989 J. Immunol. 143: 4117) and IgG (Monteiro, R. C. et al., 1992 J. Immunol, 148: 1764) classes have been developed against Fc.alpha.R.